- Glasgow-haskell-users

GHCi debugger status
by Daniil Elovkov 01 Dec '08

01 Dec '08

Hello folks I'd like to know, how do ghc developers and users feel about the debugger? I'm using it to some extent on ghc 6.8.2 and find it useful. But I'm getting an impression that it's not too stable. I'm not filing any bug report yet, just want to know how it feels for others. I used to make it panic. I think, it was due to existential types. Now I see it mess up the list of bindings in a funny way. For example, in a previous trace session I had a variable, say, prev. It was bound during pattern matching in a function, say, prevFunc. Now I'm having another trace session, actually stepping from the very beginning. A couple of steps after the beginning, prev suddenly appears in the bindings where prevFunc absolutely has not yet been invoked. It's completely unrelated. In 'show bindings' prev has a wrong type - SomeType (it's ADT). Its real type (when in scope) is [t]. I ask 'length prev' and get 0 :) So, what is your impression of the debugger? -- Daniil Elovkov

6 24

cross module optimization issues
by John Lato 28 Nov '08

28 Nov '08

Hello, I have a problem with a package I'm working on, and I don't have any idea how to sort out the current problem. One part of my package is in one monolithic module, without an export list, which works fine. However, when I've started to separate out certain functions into another module, and added an export list to one of the modules, which dramatically decreases performance. The memory behavior (as shown by -hT) is also quite different, with substantial memory usage by "FUN_2_0". Are there any suggestions as to how I could improve this? Thanks, John

6 15

GADT Type Checking GHC 6.10 versus older GHC
by Jason Dagit 28 Nov '08

28 Nov '08

Hello, Here is an example where ghc 6.8.x was fine, but now 6.10 complains. \begin{code} type CommuteFunction = forall x y. (Prim :< Prim) C(x y) -> Perhaps ((Prim :< Prim) C(x y)) commute_split :: CommuteFunction commute_split (Split patches :< patch) = toPerhaps $ do (p1 :< ps) <- cs (patches :< patch) case sort_coalesceFL ps of p :>: NilFL -> return (p1 :< p) ps' -> return (p1 :< Split ps') where cs :: ((FL Prim) :< Prim) C(x y) -> Maybe ((Prim :< (FL Prim)) C(x y)) cs (NilFL :< p1) = return (p1 :< NilFL) cs (p:>:ps :< p1) = do p1' :< p' <- commutex (p :< p1) p1'' :< ps' <- cs (ps :< p1') return (p1'' :< p':>:ps') commute_split _ = Unknown \end{code} Now with ghc 6.10 we get this error: GADT pattern match with non-rigid result type `Maybe a' Solution: add a type signature In a case alternative: p :>: NilFL -> return (p1 :< p) In the expression: case sort_coalesceFL ps of { p :>: NilFL -> return (p1 :< p) ps' -> return (p1 :< Split ps') } In the second argument of `($)', namely `do (p1 :< ps) <- cs (patches :< patch) case sort_coalesceFL ps of { p :>: NilFL -> return (p1 :< p) ps' -> return (p1 :< Split ps') }' You can see more context here: http://darcs.net/src/Darcs/Patch/Prim.lhs around line 472 My understanding was that from 6.6 to 6.8, GADT type checking was refined to fill some gaps in the soundness. Did that happen again between 6.8 and 6.10 or is 6.10 being needlessly strict here? Thanks, Jason

4 14

simple ghci debugger question
by Peter Hercek 27 Nov '08

27 Nov '08

Is there a way to redirect output of a ghci debugger command so that I can process it with a (ghci) script before it is displayed? Peter.

3 2

How to reliably terminate a process
by Bernd Holzmüller 27 Nov '08

27 Nov '08

Hi, I am writing an application on Windows XP using ghc 6.10.1 that manages test cases (same applies to ghc 6.8.3). For each selected test case, the application runs an executable associated with the test case using System.Process.runInteractiveCommand. These processes have to be aborted in some cases, for example, when their specified termination time is up. As I realized, however, aborting these processes using terminateProcess is not reliable. Most often, the processes do not terminate correctly but have to be killed from the Windows Task Manager. In some cases, the process still produces output - in particular if I apply getProcessExitCode on the process just killed. By the way: calling and killing the process from the shell with Ctrl-C successfully kills the process. Any advice on how to reliably terminate a process on Windows? Thanks, Bernd

4 3

Fun with type functions
by Simon Peyton-Jones 27 Nov '08

27 Nov '08

Friends GHC has embodied data type families since 6.8, and now type synonym families (aka type functions) in 6.10. However, apart from our initial papers there isn't much published material about how to *use* type families. But that hasn't stopped you: quite a few people are using them already, and of course there is a rich seam of work on using functional dependencies to express type-level computation. Ken Shan and Oleg Kiselyov and I are collaborating to write a paper for an upcoming workshop, under the general rubric of "Fun with type functions" (in homage to Thomas Hallgren's paper "Fun with functional dependencies" and Ralf Hinze's paper "Fun with phantom types"). So this message is to ask you: can you tell us about the most persuasive, fun application you've encountered, for type families or functional dependencies? Simple is good. It doesn't have to be elaborate: just something that does something useful you could not have done otherwise. Pointers to email threads are fine. Don't assume we already know about them (even if we participated in the thread :-) Part of what we're interested in is that *you* found the example compelling. Many thanks Simon, Ken, Oleg PS: I'm broadcasting this message to GHC-users and Haskell-cafe, but to avoid deluging ghc-users, please reply just to us and Haskell cafe. (Interested ghc-users can follow the threads there from the archives if they want.)

1 0

Linking to Haskell code from an external program
by Colin Paul Adams 26 Nov '08

26 Nov '08

I am trying to call a Haskell function from an Eiffel program, using C as an intermediary. For starters, I compiled and ran a variation of the program shown in http://haskell.org/haskellwiki/Calling_Haskell_from_C, to make sure I had the C-code right. I then attempted to move it into Eiffel. I can compile the C code OK, but I'm running into problems with linking. I solved most of the problems by adding the -v flag to the call to ghc which I used to link the original (haskell + c only) program, and cut-and-paste the linker options from their into the Eiffel configuration file. This isn't really satisfactory - I would like some automatic way to determine what the flags should be. The remaining problem has me stumped. I am getting the following messages: big_file_C4_c.c:(.text+0x9f4f): undefined reference to `__stginit_Fib' /home/colin/fib/Fib_stub.o: In function `stginit_export_Fib_zdffibonaccizuhszuaQO': Fib_stub.c:(.text+0x5): undefined reference to `Fib_zdffibonaccizuhszuaQO_closure' /home/colin/fib/Fib_stub.o: In function `fibonacci_hs': Fib_stub.c:(.text+0x32): undefined reference to `Fib_zdffibonaccizuhszuaQO_closure' The Haskell code looks like this: >{-# LANGUAGE ForeignFunctionInterface #-} > > module Fib where > > import Foreign.C.Types > import CString > > fibonacci :: Int -> String > fibonacci n = show (fibs !! n) > where fibs = 0 : 1 : zipWith (+) fibs (tail fibs) > > fibonacci_hs :: CInt -> IO CString > fibonacci_hs = newCString . fibonacci . fromIntegral > > foreign export ccall fibonacci_hs :: CInt -> IO CString And the C code (in an Eiffel wrapper) looks like this: haskell_fib (n: INTEGER): POINTER is -- String representation of `n'th Fibonacci number as a string external "C inline use <stdio.h>,%"HsFFI.h%"" alias "[ #ifdef __GLASGOW_HASKELL__ #include "Fib_stub.h" extern void __stginit_Fib ( void ); #endif int argc; char *argv[0]; argc = 0; hs_init(&argc, &argv); #ifdef __GLASGOW_HASKELL__ hs_add_root(__stginit_Fib); #endif return (fibonacci_hs($n)); hs_exit(); ]" end I can't find any mention of the word "fibonacci" in the output from the ghc run, and I don't know where this _closure stuff is coming from. Can you help? -- Colin Adams Preston Lancashire

4 12

Grammar for (Glasgow) Haskell
by Colin Paul Adams 26 Nov '08

26 Nov '08

Is this available as a happy file (or something similar)? I tried searching the 6.10.1 source package for *.y, but the three files I found don't seem to be what I'm looking for. What I want to try to do is to generate C wrappers to call GHC-compiled code by parsing foreign export declarations. But I figure just searching the code for a regular expression on the lines of foreign[ ]+export is going to be over-simplistic. But I'd rather not type in all the Haskell grammar myself (I'm lazy - you wouldn't expect any other sort of person on this list, I assume :-) ). -- Colin Adams Preston Lancashire

3 3

Exporting an #include from one package to another?
by Conal Elliott 26 Nov '08

26 Nov '08

Is there a way to package up an #include like to share across (be #include'd into) various other haskell packages? I have some standard templates for type class instances that I can't define as instances without creating lots of overlapping instances. I can write an include file easily enough, but I don't know how to get other packages to find and #include that include file. - Conal

2 2

Future plans: unicode and line editing
by Ian Lynagh 25 Nov '08

25 Nov '08

Hi all, We've been weighing up the options to solve the recent problems that editline has given us, and we think that this is the best way forward: For 6.12: * http://hackage.haskell.org/trac/ghc/ticket/2811 Implement unicode support for text I/O (we've had this on the TODO list for some time now; it was slated for 6.10, but didn't make it) * http://hackage.haskell.org/trac/ghc/ticket/2812 Switch to using haskeline instead of editline Haskeline should be able to use the above unicode support, rather than the utf8-string package * http://hackage.haskell.org/trac/ghc/ticket/2813 Create (and ship with GHC) a utf8 bytestring-a-like I'm not sure if this is actually needed by either of the above, but it is certainly related, and something we want to do anyway We won't be trying to get any of the above into future 6.10 releases. However, for 6.10 you can use Judah's ghci-haskeline: http://code.haskell.org/~judah/ghci-haskeline/ which uses the GHC API and the haskeline package to implement the GHCi frontend. If you have any problems using ghci-haskeline, then it is likely that GHCi in 6.12 will have the same problems, so please let us know! Comments and criticisms welcomed! Thanks Ian

2 2